FIG. 1 is a circuit diagram of a conventional self oscillation-type digital audio amplifier. Referring to FIG. 1, the conventional self oscillation-type digital audio amplifier includes a power PMOS transistor (PM), a power NMOS transistor (NM), a first filter 11, a second filter 12, a voltage divider 13, an error amplifier 14, a comparator 15, and a gate driver 16.
The operation of the conventional self oscillation-type digital audio amplifier of FIG. 1 will be described in the following paragraphs.
An output voltage VOUT is divided by the voltage divider 13 at a constant rate, and a result of the division, i.e., a feedback voltage VFB, is compared with an input audio voltage VIN by the error amplifier 14. Error amplified by the error amplifier 14 is converted into a pulse signal by the comparator 15 having hysteresis. The pulse signal is transmitted to the power transistors PM and NM via the gate driver 16 and then is finally used for controlling the output voltage VOUT.
More specifically, if the input audio voltage VIN is larger than the feedback voltage VFB, the PMOS transistor PM is turned on so that the output voltage VOUT increases. If the output voltage VOUT keeps increasing until the feedback VFB becomes larger than the input audio voltage VIN, the NMOS transistor NM is turned on so that the output voltage VOUT decreases.
The conventional self oscillation-type digital audio amplifier operates in an oscillation manner where the output voltage VOUT alternately increases and decreases at high speed centering around a result of amplifying the input voltage VIN at a predetermined rate. The output voltage VOUT has a waveform obtained by adding a voltage ripple generated in self-oscillation to a result of amplifying the input signal VIN having a voice bandwidth of 20 Hz˜20 kHz at a predetermined rate. Here, the voltage ripple is a voltage component having an amplitude of about 100 mV and having a higher frequency than voice signals. In the meantime, since the power switches, i.e., the power transistors PM and NM, automatically operate while the conventional self oscillation-type digital audio amplifier undergoes self-oscillation, self-oscillation frequency can be simply called switching frequency.
In the conventional self oscillation-type digital audio amplifier of FIG. 1, a resistor R1 and a capacitor C1 in the first filter 11 are connected to each other in cascade in order to sense the variation of the output voltage VOUT. Accordingly, the variation of voltage at the capacitor C1 is proportional to the amount of current passing through the capacitor C1. A difference between voltages respectively at either end of the resistor R1 corresponds to the variation of the voltage at the capacitor C1. Therefore, as large a waveform as the variation of the voltage at the capacitor C1 can be obtained by increasing the resistance of the resistor R1.
The conventional self oscillation-type digital audio amplifier can feed back the output voltage VOUT and its variation by sensing the variation of the voltage at the capacitor C1 taking advantage of the above-described structure of the first filter 11 in which the resistor R1 and the capacitor C1 are connected to each other in cascade. However, a voltage ripple at the output node (VOUT) of the capacitor C1, may undesirably increase. In order to prevent this problem, excessive voltage ripples should be compensated for by connecting the first filter 11 and the second filter 12 in cascade.
Alternatively, voltage ripples at the output node (VOUT) can be sufficiently compensated for by providing the first filter 11 without the resistor R1. In this case, however, self-oscillation frequency may dramatically decrease.